There are different devices known in the art for improving power quality and reliability of grid or utility line power supplied to sensitive loads. Three of these include, a static compensator (STATCOM), an on-line uninterruptible power supply (UPS), and an off-line UPS. A UPS may also be operated in cooperation with a power generator or gen set, for long term interruptions.
FIG. 1A illustrates a typical STATCOM 100 in which a Voltage Source Converter (VSC) 102 is connected to an AC system 104 supplying a load 106 through a shunt-connected transformer 107. A capacitor 109 is connected to the DC terminals 108 of the VSC 102 and is usually an integral part of the VSC 102. The VSC 102 controls the line voltage by injecting or absorbing reactive power. A STATCOM aids in controlling load voltage fluctuations that result from a load's transient or changing reactive power requirements. While a STATCOM has relatively low operation costs, a STATCOM does not provide active power and therefore fails to operate under short circuit conditions or other conditions where active power provision is required or desired. Further, a STATCOM has a limited ability to correct voltage fluctuations due to grid faults or switching events.
FIG. 1B illustrates a block diagram of the main components of an exemplary on-line UPS system 110. This UPS system 110 is on-line during normal operation, where on-line operation includes converting energy from a grid or utility 112 through a rectifier 114 from AC to DC, maintaining a battery 116 at full charge, and converting the energy through an inverter 120 to an AC-system resulting in double conversion. A static bypass switch 123 and the mechanical bypass switch 124 are normally open. The UPS system 110 typically operates such that it is synchronized with the bypass source 126 or with the grid 112. A chemical battery 116 is used as energy storage for bridging outages. In case of a malfunction of the system 110 the mechanical switch 124 allows operation by connecting the grid 112 or bypass supply 126 directly to the load 130. In case of a malfunction on the load assembly, the static bypass switch 123 is closed to increase short circuit capability for fuse coordination. In case of a malfunction on the grid, the rectifier 114 is blocked and energy is taken from the battery 116 without disturbances on the load 130.
The on-line UPS 110 requires double conversion resulting in relatively low efficiency and high operation costs. Further, the grid 112 is decoupled from the load and, thus, there are no transients on the load voltage under grid disturbances. Short circuit capability is provided by closing the static bypass switch 123.
FIG. 1C illustrates a block diagram of the main components of a typical off-line UPS 140. The off-line UPS 140 is off-line during normal operation, where off-line operation provides that a solid state breaker (SSB) 122 is closed, a mechanical bypass switch 124 is open, and a static converter 142 maintains a battery 116 at full charge. A chemical battery 116 is typically used as energy storage for bridging outages. Outage and sag conditions on the grid must be detected and compensated for fast in order to protect the load or load assembly 130. In case of a malfunction on the load assembly 130, the SSB 122 remains closed to make use of the grid short circuit capability for fuse coordination. In case of a malfunction of the off-line UPS system 140, the mechanical bypass switch 124 allows operation by connecting the grid 112 directly to the load 130. In case of a malfunction on the grid, the SSB 122 will be opened and the converter 142 supplies the load. The off-line UPS 140 operates at relatively low operation costs. The grid 112 is coupled to the load 130, thus, grid disturbances are transferred to the load 130 under standby conditions (normal operation) until the SSB opens.
FIG. 2 shows a block diagram of a UPS system 148 having an off-line UPS 150 in cooperation with a power generator or gen set 152 for long term interruptions. The off-line UPS 150 consists of a converter 154 and an energy storage device 116, such as, for example, a chemical battery, an array of chemical batteries, or other storage devices or systems. The converter 154 provides fast dynamic behavior. The converter power semiconductor(s), typically used in the converter, however, have no or substantially no overload capability. An accompanying UPS control system (not shown) provides for operation of the switch 156 in the event of an outage or sag, and for proper charging of the battery.
For long term interruptions the independent gen set 152 is connected directly to the load side of the AC-system. The gen set 152 consists of a power source (such as, for example, a natural gas, diesel engine, gasoline engine or other engine) and a mechanical to electrical conversion device (i.e. a generator). An accompanying gen set control system (not shown) controls the torque and the speed of the shaft producing active power. In the conventional off-line IPS with an independent gen set, the gen set control system does not cooperate with the UPS control system. The shaft speed (for example revolutions per second) routinely corresponds to the electrical system frequency (for example, cycles per second or hertz). Typically, the gen set has a long response time to dynamic voltage (or current) variations and a large overload capability. The long response time is a result of the electromechanical and the power generation process with its rotating mass or momentum. System resonance frequencies in the area of a few Hertz are usual.
The gen set 152 and the UPS 150 each typically have their own independent closed loop control unit (not shown). The operation principle of the UPS system 148 provides for the operation of the two independent gen set 152 and UPS 150 devices in the following way:
1. Standby Mode: gen set 152 is not in operation, and UPS 150 is in standby mode, but is not exchanging power with the load (it may be maintaining the storage charge). System 148 control system (not shown) monitors the grid voltage. The switch 156 is closed.
2. Disturbance on the grid side: The system 148 initiates the switch 156 to open; the load 130 is taken over by the UPS 150 (island mode); and depending on the energy content of the storage device 116, the gen set 152 is started.
3. If the interruption is only short term: the system 148 initiates the switch 156 to close; load 130 is handed over to the grid 112; the storage device 116 is charged; and transfer back into standby mode; gen set 152 is not in operation.
4. If the interruption is long term: the system 148 transfers from the UPS 150 to the gen set 152; UPS system 150 remains in standby mode; and switch 156 is open.
5. When the long term interruption ends: the system 148 initiates the switch 156 to close; and the load 130 is transferred to the grid 112.
In the case of a long term operation, the gen set 152 provides the active power to the load. The operation of this UPS system 148 allows for one of either the gen set 156 or the UPS 150 to operate at any given time. There is no common control or coordination or simultaneous operation, only separate individual control of the gen set and UPS with sequential operation of each. The drawback of this operation principle is that the good dynamic behavior of the UPS (that is, rapid response to reactive or active power variations in the load and stabilization of frequency), especially during generator operation, cannot be used or achieved because there is no common control unit available.
The standby gen set provides limited dynamic behavior during start up, even if it is a diesel gen set (DGS) of the fast starting/running type (for example, of the type running at 1800 rpm), with lubrication system heating. In these systems the starting phase may typically last 5 to 8 seconds until nominal speed (no-load) has been reached after which one may switch in load elements.
DGS load connection or switch-in as well as rejection produce speed and frequency deviations requiring at least 2 to 5 seconds until frequency deviation has reached a steady state value (for example, made up of control dead time, fuel injection time constant and settling time). In the case of start up, this period must be added to the 5 to 8 seconds required for attaining nominal speed, as described above so that the total time for startup and stabilization may be at least 7 to 13 seconds or more. FIG. 3 shows a typical frequency 160 behavior during load switching. Typical frequency deviation resulting from speed deviation caused by load change (switch-in and rejection) of a standby diesel gen set. The so-called dynamic deviation depends on the:
Inertia (the rotating mass of the engine): small inertia large excursion.
Turbo-charging: the higher the charging degree the larger the deviation.
Size of loads subject to switching: the larger the load size the larger the deviation.
Frequency deviations (typically approximately 10% of rated frequency, or about .+−0.6 Hz for 60 Hz operation or .+−0.5 Hz for 50 Hz operation) lasting several seconds may cause trouble or even damage to frequency dependent loads like computer screens, TV sets and other such devices. Note that in nominal conventional grid connected power systems the steady state frequency deviation is held within .+−.0.1 Hz and that at frequencies below 58.5 Hz (that is, at a drop of 1.5 Hz) of system frequency (60 Hz) load shedding occurs.
Load takeover normally takes place stepwise (for example, in three 3 steps) due to the limited size of the diesel and gen-set (for economic reasons the equipment is usually sized not much larger than the load) as well as due to drastic speed deviations (and therefore drastic frequency excursions), as discussed above.
In order to avoid the frequency deviations and load takeover restrictions described above, system designers could specify a gen set size that is as much as five times larger than the load, with resulting economic penalties.
If a voltage source converter (VSC) is employed for the converter 154 then STATCOM operation is possible, exchanging reactive power, and absorbing active power to cover the losses. However, in conventional UPS plus gen set systems, the full functional advantages of using a four-quadrant voltage source converter (VSC) during transfer to and subsequent operation of the gen set are not realized at least since the individual UPS and gen set controls are not cooperated. Conventional control systems do not provide system designers as much incentive or flexibility to choose the VSC for the converter (see FIG. 2) since the VSC's full capabilities are not engaged by the available conventional control system technology.